Simulator cart

ABSTRACT

Methods and apparatus are provided which allow control systems to be tested. Specifically, a test system is coupled to a control system in a manner which allows the test system to communicate with and drive the control system by sending and receiving signals via both the controller-I/O communication channel and the field I/O connectors. In essence, the test system is used to both simulate a plant to be controlled and to monitor, validate, and or modify the internal state of the control system controller and possibly the control system I/O interface. Plant simulation is accomplished by simulating the I/O devices to which the control system is coupled (and hence the plant processes) when installed in its operational environment. In addition to the simulation of I/O devices, the test system takes advantage of the fact that many commonly used controller and I/O interfaces are capable of communication with other devices by using such communications ability to provide instructions to or obtain information from a control system&#39;s controller(s) and I/O interface(s).

This application is a divisional of applications

Application Ser. No. 09/936,080 filed on Mar. 12, 2002 now U.S. Pat. No.6,904,380.

International Application PCT/US00/07962 filed on Mar. 23, 2000, andwhich designated the U.S.

The nonprovisional application designated above, namely applicationPCT/US00/07962, filed Mar. 23, 2000, claims the benefit of U.S.Provisional Application No. 60/126,060 filed Mar. 25, 1999.

This application claims the benefit of U.S. provisional application No.60/126,060 incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The field of the invention is test and simulator systems.

BACKGROUND OF THE INVENTION

Control systems incorporating PLCs (Programmable Logic Controlers) andDCSs (Distributed Control Systems) are frequently used to control realworld processes by accepting inputs which typically originate fromsensors such as, among others, those used to sense temperature, flow,level, radiation, light, movement, and pressure, and those used ingenerating outputs which are used to drive actuators such as hydraulicdevices, valves, lights, and motors. Control systems can often be viewedas having a control component and an interface component, with one orboth components having both hardware and software sub-components. Thus aPLC based device might utilize a digital PLC having embedded software asthe control component (the “controller”), with an interface component(the “I/O interface”) (a) accepting signals from sensors and convertingthem into a form acceptable to the PLC, and (b) accepting outputs fromthe PLC and converting them to signals suitable as inputs to theactuators. In such systems, the controller and I/O interface are oftenconnected by one or more paths (the “controller-I/O communicationchannel) to allow communication and control signals to pass between thecontroller and the I/O interface. Similarly, the I/O interface is, afterthe control system is installed in its operating environment, connectedvia one or more electrical paths (the “field wiring”) to the componentsfrom which the control system receives its inputs, and to the componentsto which the control system directs its outputs, with the I/O interfacebeing provided with a plurality of connectors (“field I/O connectors”)which facilitate connecting the I/O interface with the field wiring.Many control systems will also incorporate a human-machine-interface(HMI) component comprising hardware and or software for facilitatingoperator interaction with the control system.

Although methods and devices for testing control systems are known, theygenerally have individual strengths and weaknesses which make them moreappropriate in some situations and less appropriate in other. Thus,there is a continuing need for improved methods and apparatus forcontrol system testing.

SUMMARY OF THE INVENTION

Methods and apparatus are provided which allow control systems to betested. Specifically, a test system is coupled to a control system in amanner which allows the test system to communicate with and drive thecontrol system by sending and receiving signals via both thecontroller-I/O communication channel and the field I/O connectors. Inessence, the test system is used to both simulate a plant to becontrolled and to monitor, validate, and or modify the internal state ofthe control system controller and possibly the control system I/Ointerface. Plant simulation is accomplished by simulating the I/Odevices to which the control system is coupled (and hence the plantprocesses) when installed in its operational environment. In addition tothe simulation of I/O devices, the test system takes advantage of thefact that many commonly used controller and I/O interfaces are capableof communication with other devices by using such communications abilityto provide instructions to or obtain information from a control system'scontroller(s) and I/O interface(s).

It is contemplated that the methods and apparatus disclosed herein willprovide an effective approach to connecting a control system to betested to simulated I/O devices and controlling and monitoring thesimulated I/O devices so as to produce inputs to the control system, torecord outputs from the control system, and to compare the recordedoutputs against an expected value and record the result of thecomparison for reporting purposes.

It is also contemplated that the methods and apparatus disclosed hereinwill provide an effective approach to the staging and testing ofPLC/HMIs (Programmable Logic Controllers/Human Machine Interfaces) andDCSs (Distributed Control Systems) before delivery to the field.

It is also contemplated that the methods and apparatus disclosed hereinwill provide an effective mechanism for validating the operation of acontrol system once it has been installed in a plant to be controlled.

Various objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments of the invention, along with theaccompanying drawings in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a simulator cart embodying the invention.

FIG. 2 is an alternative schematic view of the simulator cart of FIG. 1.

FIG. 3 is a schematic view of a simulator coupled to a plant controlsystem.

FIG. 4A is a diagram of a preferred layout for components on a firstside of a cart based simulator.

FIG. 4B is a diagram of a preferred layout for components on a secondside of the simulator of FIG. 4 a

FIG. 5A is a wiring diagram illustrating a preferred method ofinterconnection for the components of the simulator of FIG. 4A.

FIG. 5B is a wiring diagram illustrating a preferred method ofinterconnection for the components of the simulator of FIG. 4B.

FIG. 6 is a diagram of a preferred layout for components in a“briefcase” based simulator.

FIG. 7 is a screen capture of a simulator HMI screen wherein an operatoris being prompted to select which test section to run.

FIG. 8 is a screen capture of a simulator HMI screen wherein an operatoris being prompted to select whether to run a particular test or anentire test procedure.

FIG. 9 is a screen capture of a simulator HMI screen wherein an operatoris being prompted to perform a first test.

FIG. 10 is a screen capture of a simulator HMI screen wherein theoperator is being prompted to enter the results of the test of FIG. 9.

FIG. 11 is a screen capture of a simulator HMI screen wherein anoperator is being prompted to perform a second test.

FIG. 12 is a diagram of a simulator HMI database table.

FIG. 13 is a diagram of a simulator HMI database table.

FIG. 14 is a diagram of a simulator HMI database table.

FIG. 15 is a diagram of a simulator HMI database table.

FIG. 16 is a diagram of a simulator HMI database table.

FIG. 17 is a diagram of a simulator HMI database table.

FIG. 18 is a diagram of a simulator HMI database table.

FIG. 19 is a diagram of a first method embodying the invention.

FIG. 20 is a diagram of a second method embodying the invention.

FIG. 21 is a diagram of a third method embodying the invention.

FIG. 22 is a diagram of a fourth method embodying the invention.

FIG. 23 is a diagram of a fifth method embodying the invention.

FIG. 24 is a diagram of a sixth method embodying the invention.

FIG. 25 is a diagram of a seventh method embodying the invention.

FIG. 26 is a diagram of an eighth method embodying the invention.

DETAILED DESCRIPTION

Referring to FIGS. 1–3, a simulator cart test system (“SIMCart”) 10 iscoupled to a control system to be tested (“target CS”) 20. SIMCart 10includes rolling enclosure 110, I/O simulator modules 120,HMI/configurator 130, hardcopy device 140, wiring harnesses 150, andcommunication channel 160 connections 161 and 163. Target CS 20 includesenclosure 210, I/O interfaces 220, field I/O connectors 225, HMI 230,communication channel 160 connections 261 and 263, and PLC 270. SIMCart10 uses wiring harnesses 150 to send and receive I/O signals to and fromI/O interfaces 220 so as to simulate the input devices/sensor 50 andoutput devices/actuators 40 which would be connected via the fieldwiring 41 and 51 to target CS 20 when target CS 20 is installed. SIMCart10 uses communications channel 160 to obtain register and statusinformation from I/O interfaces 220, HMI 130 and PLC 270. By monitoringthe internal state of and data flow between I/O interfaces 220, HMI 130and PLC 270, SIMCart 10 is better able to isolate any problems which mayoccur in target CS 20 during testing as occurring in a particularsubsystem of target CS 20. The manner in which SIMCart 10 is coupled totarget CS 20 also permits SIMCart 10 to test multiple I/O channels andor subsystems of target CS 20 simultaneously.

The SIMCart is automatically initialized by utilizing communicationschannel 160 to query target CS 20 to obtain information which allowsSIMCart 10 to perform automatic initialization and mapping of its inputand outputs to those of target CS 20. SIMCart 10 incorporates aknowledge base which includes characteristics of known control systemhardware and software components in performing such automaticinitialization. It is contemplated that the knowledge base might beextended to include mathematical models of standard components such assensors and actuators to allow SIMCart 10 to better simulate suchcomponents and to allow automatic test generation for target systemswhich will be coupled to such components. FIGS. 12–18 show some of thefields and data which might be used in maintaining such a knowledgebase.

SIMCart 10 is contemplated as being beneficial in the development andoff-site testing of target CS 20 by simulating the plant which target CS20 is intended to control. If the capacity of a single SIMCart 10 isinsufficient to simulate an entire plant, multiple SIMCarts 10 can beused to achieve the simulation. Such a combination of SIMCarts 10 mayutilize multiple HMI 130 devices, or may share a single HMI 130. SIMCart10 is also contemplated as being beneficial for off-site training ofpersonnel in the operation of target CS 20 when SIMCart 10 is used inconjunction with target CS 20. Such off-site use of SIMCart 10 iscontemplated as providing, among others, complete testing of all targetCS 20 inputs and resulting outputs before field installation

On-site testing is also contemplated as benefiting from the use ofSIMCart 10, either in its entirety or only its configurator/HMI portion.Once target CS 20 is installed in its operational environment,configurator 130 can be used as a test and validation tool. Theconfigurator 130 can provide step-by step instructions to a field testerwhich, if followed, result in a desired level of testing of target CS20. By doing so, configurator 130 essentially controls the test processthrough the field tester. By requiring feedback from the field testerupon the completion of various portions of the testing, a test reportwhich includes the tests steps performed and the tester responses can begenerated so as to provide assurance that the required level of testinghas be completed. Such “controlled” testing thus provides a method ofrepeatable, validated and verified testing. Configurator 130 can also beused to record the results of such testing and to generate reports inthe field which provide feedback as to the current state of target CS 20and the field wiring to which target CS 20 is coupled.

If the entire SIMCart 10 is used on-site, SIMCart 10 can be interchangedwith selected inputs and outputs of target CS 20 so as to allow “live”inputs to cause target CS 20 to control simulated actuator components,or to use simulated inputs to cause target CS 20 to control “live”actuator components, or to mix and match live and simulated inputs andoutputs as desired. If configurator 130 is still attached to the rest ofSIMCart 10 during testing, and if SIMCart 10 is coupled to target CS 20during testing, configurator 130 can control the test process boththrough direct interaction with target CS 20 and through a test operatoras previously discussed.

Rolling enclosure 110 of SIMCart 10 is preferred to be sized anddimensioned to house I/O simulator modules 120 and any necessarysupporting systems in a manner which facilitates the use of SIMCart 10and its interconnection with target CS 20.

I/O simulator modules 120 may comprise PLCs or smart I/O systems. TheSIMCart 10 of FIG. 1 utilizes several Sixnet I/O related productsincluding ST-DI-024-16H, ST-PS-024-02N, ST-DO-DC2-16H, ST-AI-20M-16H,ST-AO-20M-08F, ST-DI-120-08U, ST-DO-AC1-08U, and ST-GT-ETH-24P. Detailsof the preferred Sixnet products are attached to the provisionalapplication as part of its disclosure, said provisional applicationhaving previoiusly been incorporated by reference herein. FIGS. 4A and4B provide a diagram of a preferred layout of components in SIMCart 10,and FIGS. 5A and 5B provide corresponding wiring diagrams showing apreferred method for interconnecting the components of SIMCart 10.

An alternative embodiment utilizes a smaller number of components sothat a SIMCart type simulator can be fit within a carrying case. FIG. 6provides a diagram of a preferred layout of components in such anembodiment.

HMI/configurator 130 of SIMCart 10 is contemplated as being a generalpurpose computer such as a laptop PC (“PC”). Portability is particularlydesirable as it facilitates the transportation of HMI 130 into the fieldto support on-site testing of an installed target CS 20. HMI 130 iscontemplated as incorporating software which allows testing of alarmresponses (including deadbands), loops, and logic, and reporting of HMIreadouts, trend graphs, alarms, reports, system requirements, etc. Testresults are stored in the form of textual date and time stamped data,graphs, and tester responses to prompted questions.

In a preferred embodiment, HMI 130 includes a multi-tasking operatingsystem, preferably Windows NT™, which is capable of supporting agraphical user interface (“GUT”) and a relational database system(preferably Microsoft Access™), as well as software for controlling I/Osimulator modules 120, and testing target CS 20. HMI 130 is alsopreferred to include an Ethernet network interface card for connectionto communication channel 160 which is preferred to be an TCP/IP basedEthernet network. It is also preferred that HMI 130 of SIMCart 10 bedetachable from the rest of SIMCart 10 to allow it to be usedindependently as an onsite verification tool.

HMI/configurator 130 provides for downloading test relational databases,initiating tests and capturing tester responses and comments, andgenerating reports. The test databases include test cases, expectedresults for test comparison and exception generation. An overall testplanner database organizes the sequence of test events. Being as thetest databases are derived from the requirements database, the systemowner is assured of lock-step configuration control and traceablerequirements testing.

It is preferred that configurator 130 utilize a relational databasecoupled to a GUI to allow entry of and storage of test requirements at ahigh level, and to provide tools for deriving lower level sets of testinstructions from such high level requirements in a manner whichmaintains a correlation between lower level test sets and higher levelrequirements. By maintaining such a correlation, changes to high levelrequirements can be quickly, and sometimes automatically, incorporatedinto the lower level test sets. Such lower level test sets will be usedto drive and control both automatic and interactive testing of target CS20. By maintaining a correlation between high level and low levelrequirements, reports showing the test steps used to verify satisfactionof the high level requirements through performance of the low level testsets can be generated.

It is contemplated that the relational database used by configurator 130to maintain test requirements and instructions should provide for thespecification of: (1) initial values, an operational range, a deadband,and a physical range for each I/O component to be tested to allow thetesting of alarms when values are within and outside of such ranges; (2)standardized questions for prompting the operator to perform variousfunctions or to provide responses; (3) sets of instuctions and questionsfor interactive testing (or possibly automatic testing) of the HMIportion of target CS 20; (4) parameters for PID loop testing; (5)parameters for indicator testing; (6) general target systemrequirements, as well as supporting any other tables required for properfunctioning of SIMCart 10. FIG. 7 provides a screen capture image of asimulator HMI screen wherein an operator is being prompted to select atest section to run. FIG. 8 provides a screen capture image of asimulator HMI screen wherein an operator is being prompted whether torun a particular test or an entire test procedure. FIG. 9 provides ascreen capture of a simulator HMI screen wherein an operator is beingprompted to perform a test. FIG. 10 provides a screen capture of asimulator HMI screen wherein an operator is being prompted to enter theresult of the test of FIG. 9. FIG. 11 provides a screen capture of asimulator HMI screen wherein an operator is being prompted to performanother test.

Hardcopy device 140 is preferred to be a printer suitable for connectionto HMI 130, and for printing hardcopy of tests, test results, and auditinformation as well as other reports discussed herein.

Wiring harnesses 150 may take on almost any form so long as theyproperly transmit I/O signals between SIMCart 10 and target CS 20.However, it is preferred that wiring harnesses 150 be chosen tofacilitate the coupling of target CS 20 to SIMCart 10 and the simulationof the field wiring to which target CS 20 will be coupled in itsoperational environment. It is contemplated that several differentharnesses may be included with the SIMCart 10 with each harness beingdesigned to facilitate the coupling of SIMCart 10 to a particular typeof target CS 20. The inclusion of such pre-configured harnesses iscontemplated as facilitating the use of SIMCart 10 to test a variety ofstandard control systems such as Allen-Bradley PLC-5 controllers orSiemens S7 controllers.

Communication channel 160 is preferred to be a TCP/IP based Ethernetnetwork and is preferred to be shared between SIMCart 10 and target CS20 to allow flexible interaction between HMI 130, I/O simulator modules120, HMI 230, PLC 270, and I/O interface 220. However, it iscontemplated that other communication methods such as RS-232 interfaces,token ring networks, FDDI networks, proprietary control busses, etc. mayalso be used to provide a communication channel between one or morecomponents of SIMCart 10 and target CS 20.

As previously discussed, the HMI/configurator portion 130 of SIMCart 10is useful both in the operation of SIMCart 10, but also as a detachablevalidation and/or training tool. It is contemplated thatHMI/configurator 130 comprise the software and one or more databasesnecessary to allow it to perform its training and/or validationfunctions. Such functions might include, among others, the ability tocontrol access by requiring an operator to log in, the ability tosimulate a particular function and to test the capability of the targetCS to respond to a sensor's entire range of input, the ability to promptthe operator to perform operations using the target CS HMI, andautomatically performing tests against target CS when such tests do norequire manual operations by an operator.

Thus, a particular configurator embodiment is likely to have the abilityto store and run multiple tests wherein each test comprises one or moresteps. In running a particular test or sequence of test steps, theconfigurator will prompt the operator to perform a function on thetarget CS whenever manual intervention by the operator is required, butwill automatically (i.e. without operator action) run through any testssteps which do not require such manual intervention. Thus, to test apump, the configurator may prompt an operator to turn on the pump, then,after the operator has indicated to the configurator that he has turnedon the pump, the configurator will ask a question to verify properoperation of the pump such as “Did the pump indicator light turn on?”which the operator can either respond to affirmatively or negatively. Ifnegatively, the configurator will prompt the operator to enter a commentas to what response by the target CS was actually seen. During anotherstep, the configurator may generate simulated events to which the targetCS has automated responses which can be detected by the configurator andin which case the simulator will simply simulate the events and monitorthe target CS's response. As an example, if a power voltage levelexceeds a threshold amount the target CS may be required to cut off thesource of power. Such a test would be performed by the simulator at theappropriate point in a test sequence by sending an over voltage signalto the target CS and monitoring the response of target CS to see if apower cutoff signal is sent. Test results, whether operator responses orautomatically monitored events/values are also stored in one or moredatabases so that an exceptions report comparing actual target CSresponses to expected results can be generated.

It is contemplated that the configurator will comprise a scalablelibrary of test databases and test functions utilizing the testdatabases to perform the test steps. Utilizing database tables to storetests and test parameters allows a tester to add new tests and/or testscenarios, and also to modify parameters for existing tests. Suchscalable libraries and dynamic updates make for a much more robust andflexible system.

FIGS. 12–18 show some of the fields and data which might be included.The table of FIG. 12 is an “Alarm” table and includes, possibly amongothers, the fields TagName, Initial, LL_Limit, L_Limit, H_Limit,HH_Limit, and Deadband. The table of FIG. 13 is an “Alarm G” table andincludes, possibly among others, the fields Alm_Ind, QNum, Type, andQuestion. The table of FIG. 14 is an “Indication” table and includes,possibly among others, the fields ID, TagName, Description, EUZero,EUMax, and Eunit. The table of FIG. 15 is an “PID Loop” table andincludes, possibly among others, the fields PID_Number, Loop_Desc,Process_TagName, Min_EU, Max_EU, and EU_Name_SetPoint_TagName. The tableof FIG. 16 is an “Section IQ” table and includes, possibly among others,the fields ID, SectionNum, SectionStep, and Instruction. The table ofFIG. 17 is a “Section List” table and includes, possibly among others,the fields Num and Name. The table of FIG. 18 is an “SysReqs” table andincludes, possibly among others, the fields ID, Section, Requirement,and Question.

SIMCart 10 is contemplated as being particularly well suited for anumber of uses when taken to an operational facility and connected inplace of all or part of the field wiring including but not necessarilylimited to: (1) testing existing logic for reaction to new scenarios;(2) testing new logic for operations; (3) training process operatorsprior to an operational campaign to a) score and validate operatorreadiness, b) identify deficient training areas, and c) ensureoperational readiness as an overall team; (4) training process operatorsat the operational facility using by plugging SIMCart 10 in in place ofthe plant's actual sensors and actuators; (4) field trouble shooting ofwiring and other field related problems.

FIRST EXAMPLE APPLICATION

An example application of SIMCart 10 is its in revalidating a PLCprogram used to manufacture a drug wherein the PLC does so bycontrolling ingredient weighing, heating, cooling, tank level control,pump control, and other portions of the manufacturing process. Thisexample refers to target CS 20 as if it were the system to bere-validated. Revalidation of the PLC program of target CS 20 wouldinvolve first using the SIMCart 10 to perform off-site testing of targetCS 20, and then detaching the HMI/Configurator from the rest of SIMCart10 and taking it on-site in the field with target CS 20 when target CS20 is installed at the drug production facility.

During off-site/lab testing, I/O simulation modules 120 are used toprovide an input to each of the PLC I/O channels via field I/Oconnectors 225. The SIMCart automatic initialization routines create amapping between the target PLC and SIMCart I/O channels. A tester wouldlog into the SIMCart configurator computer/HMI 130 (requiring operatorsto log in facilitates future auditing). After logging in, test databasesare downloaded from a requirements database that was established fortarget CS 20 at the project start. The test databases are then used totest all functions of the PLC, HMI and network including alarms, loops,and field wiring to satisfy the testing requirement contained in thetest databases. Such testing is accomplished via a series of automatictests, as well as tests which prompt the operator to perform certainactions and to provide responses/inputs to the HMI after performing suchactions. The testing of alarms includes the testing of deadbands, andappropriate low—low, low, high, and high—high alarm ranges as well asother types of data deltas. All data can be stored electronically with adate and time stamp as well as with other audit information asnecessary.

Once off-site/lab testing is complete, the SIMCart configurator/HMI 130is datached from the SIMCart and target CS 20 and taken to the field tobe used onsite to test target CS 20 once it is installed in the drugproduction facility. In the field, the SIMCart configurator provides afield tester with a series of tests related to field I/O and wiringconnections. For some tests, the field tester is prompted by the SIMCartconfigurator to instruct a technician, possibly via cell-phone, to inputsignals to the system which will be passed to target CS 20 via the fieldwiring. The SIMCart configurator 130 then prompts the field tester toenter responses answering questions related to the feedback HMI 230 oftarget CS 20 provided to the tester in response to the technician'sinputs. All test questions and results are stored so as to allow areport of test results to be generated.

SECOND EXAMPLE APPLICATION

Once test target CS 20 is installed in the drug production facility,SIMCart 10 can periodically be used to train process operators. Suchtraining can be done on site, using the target CS 20 HMI by simplydisconnecting target CS 20 from the plant's sensors and actuators,probably by disconnecting the field wiring, and connecting SIMCart 10 totarget CS 20 in place of the sensors and actuators. Once target CS 20 isoperating with simulated I/O rather than live sensors and actuators,operators are free to interact with target CS 20 without any fear ofconsequences to the plant. When used in such a manner, the testprocedures used to validate target CS 20 may be used in trainingoperators, or else SIMCart 20 may incorporate specific software and/ordata for training operators. Such software and/or data may incorporatemulti-media presentations such as training videos. SIMCart 20 may alsowork partially or completely independently of the operator so as tosimulate events, track operator responses to the simulated events,and/or report on the quality of the operator's responses, without theoperator having to interact with SIMCart 20 during training.

Thus, specific embodiments and applications of devices and methods forplant control system testing have been disclosed. It should be apparent,however, to those skilled in the art that many more modificationsbesides those already described (including the methods shown in FIGS.8–15) are possible without departing from the inventive concepts herein.The inventive subject matter, therefore, is not to be restricted exceptin the spirit of the appended claims. Moreover, in interpreting both thespecification and the claims, all terms should be interpreted in thebroadest possible manner consistent with the context. In particular, theterms “comprises” and “comprising” should be interpreted as referring toelements, components, or steps in a non-exclusive manner, indicatingthat the referenced elements, components, or steps may be present, orutilized, or combined with other elements, components, or steps that arenot expressly referenced.

1. A method for testing a plant control system having a controller, asensor input port, an actuator output port, a series of communicationsegments communicatively coupling the sensor input port to thecontroller such that an input signal applied to the sensor input resultsin one or more signals traveling along each of the communicationsegments so as to notify the controller of the applied input signal, anda series of communication segments communicatively coupling thecontroller to the actuator output port such that a control signalgenerated by the controller results in an output signal being applied tothe actuator output port, the method comprising: providing a simulatorand coupling the simulator to the control system at at least threepoints, wherein a first point is the sensor input port, a second pointis a point from which data relating to the internal state of thecontroller are obtained, and a third point is the actuator output port;causing the simulator to apply a signal to the first point and tosubsequently obtain data on the internal state of the controller fromthe second point and to monitor the actuator output port using the thirdpoint for any applied signals; and wherein the plant control system isat least in part disconnected from a plant that is to be controlled bythe control system during testing the plant control system.
 2. Themethod of claim 1 with the control system further comprising an I/Ointerface wherein coupling the simulator to the control system at thesecond point involves tying the simulator into a network used as acommunication segment between the controller and the I/O interface. 3.The method of claim 2 wherein the I/O interface comprises at least twoanalog/digital converters with a first, input converter converting ananalog input signal applied to the sensor input by the simulator orsensor into a digital signal which is transmitted to the controller; anda second, output converter converting a digital signal received by theI/O interface from the controller into an analog signal which istransmitted to the simulator or actuator.
 4. The method of claim 3wherein the simulator obtains data on the internal state of thecontroller by establishing a network connection with the controller andrequesting that the controller report its internal state.
 5. The methodof claim 1 wherein the plant control system is located off-site relativeto the plant that is controlled by the plant control system.